In the present description, the "image generator" is denoted as any system that produces an image, such as a cathode ray tube, liquid crystal screen, plasma screen, thermoluminescence screen, hologram generator or any means of producing 2 or 3 dimensional images.
The "source image" is denoted as the image produced by the image generator; for example if the image generator is a cathode ray tube, the source image is carried on the screen of that tube.
The "axis" of the source image is the normal vector to the source image at its center.
In the case where the source image allows two distinct planes of symmetry, the source image "axis" is denoted as the intersection of these two planes, and the center of the source image is denoted as the intersection of that image with the image axis.
The "observer" is any person observing the enlarged source image through an image enlargement system.
The "thick lens" is a piece of transparent material, globally having the shape of a disk or rectangle, and bounded on each of its faces by a surface. Such a lens is not compelled to have a symmetry of revolution, and does not necessarily have a plane of symmetry.
In a section through a thick lens by a plane, it will be stated that, within the given plane, an intersection curve of the given plane and one of the faces of the lens is "convex" at a point V if, for every point W in the neighborhood of V, the segment VW is entirely within the transparent material comprising the lens. It will be stated that the given curve is "strictly convex" at V, if V and W are the only common points between the given segment VW and the surface of the cited lens.
Likewise, it will be stated that such a curve is "concave" at V, if the entire segment VW is entirely outside (outside the boundaries) of the transparent material.
A "Fresnel Lens" will be denoted as a form of achieving a thick lens, such a manufacturing method uses an echelon technique over at least a part of the so called Fresnel lens.
The "axis" of a lens is the normal vector to the lens at its center.
In the case where a lens allows two distinct planes of symmetry, the axis of such a lens is denoted as the intersection of these two planes, and the center of such a lens is denoted as the intersection of the lens with the lens axis.
The "optically useful part of the lens" is denoted as a part of the thick or Fresnel lens where light rays enter and leave, and take part in the enlargement effect of the source image.
A "Fresnel surface" is one of the two echelon surfaces of a Fresnel lens.
A "useful flank" surface is denoted as a flank where the light rays enter and leave which participate in the enlargement effect of the source image.
A "connection flank" on a Fresnel surface, is denoted as a flank where the light rays entering or leaving do not participate in the enlargement effect of the source image. Two successive useful flanks are connected either by zero connection flanks (case where two successive useful flanks have a common point), one or several connection flanks.
A "cylindrical lens" is denoted as a thick or Fresnel lens generated by a closed plane surface moving in a straight line.
A "diagonal" of an image is the distance between two points of the image that are the farthest from each other. In the case where the image is rectangular, its "diagonal" is the rectangle's diagonal.
The "effective enlargement" is the ratio between the tangent of the angle under which a linear segment of a virtual image, produced by an optical system, is viewed by an observer, and the tangent of the angle under which the linear segment of the corresponding source image would be viewed by the same observer if the optical system were absent.
The "observation zone" is the zone where the observer can be located in order to observe a correct image. In the case where the optical image possesses a symmetry of revolution, the observation zone is a cone having for its axis the lens's axis, and its peak at the center of the lens face on the observer's side. The observation zone is characterized by the "aperture" of the system, which is the half angle at the peak of the cone.
There exist optical techniques for forming a technological background plane. Their technical domain and the optical problems to resolve are not those of the present invention. U.S. Pat. No. 3,936,151 describes an optical system carrying a lens having a convex face at the center and concave at the periphery, formed according to a non-circular echelon technique, and a plane face. The light source is a point. GB-A-902 535 describes a lens producing a collimated beam of rays, coming exclusively from a point light source (focus of the lens). U.S. Pat. No. 4,423,438 describes a projection system for television images producing a real image. A Schmidt lens is used to correct for spherical aberration. This lens has a plane face. U.S. Pat. No. 3,980,399 describes a manufacturing process of a lens having a convex profile at the center and concave at the periphery, on the two lens faces.
The known systems for image enlargement are described in FR-A-1 346 696, FR-A-1 379 018, FR-A-2 472 197 and in U.S. Pat. No. 3,418,426. In these known systems a Fresnel lens is used, to produce an enlarged virtual image of a source image that can be formed by a television screen. FR-A-2 472 197 describes a Fresnel lens simulating a thick lens of revolution, where the face on the source image side is concave in the central region of the thick lens, and convex in the peripheral region, and where the face on the observer's side is everywhere convex.
The disadvantages of the system cited above are the following:
the observation zone is very reduced (except for FR-A-2 472 197); PA1 the space taken up by the system is significant, specially for FR-A-2 472 197, where a part of the lens is at a distance from the source image equal to the diagonal of the source image, and where on the other hand, the lens itself is of significant size. PA1 the system permits the enlargement of the source image to an effective enlargement of the order of 2, without notable image deformation; PA1 the observation zone is at least as wide as the FR-A-2 472 197 system; PA1 significantly less space is taken up than the system presented under FR-A-2 472 197.
In contrast, the system according to the present invention, presents the following advantages: